The present invention relates to machine tools in general, and more particularly to improvements in automatic machine tools including turret lathes and analogous machines wherein a reciprocable carriage is movable from a first or retracted position to a plurality of different second positions to thereby place a selected tool thereon into an optimum position for treatment of a workpiece which is rotated by a work spindle.
It is known to employ in an automatic lathe a carriage which is reciprocable toward and away from the workpiece in the work gripping means of a rotary spindle and which supports an indexible tool turret. It is also known to utilize springs or other biasing means for permanently urging the carriage to its retracted or first position as well as to employ an apparatus which can advance the carriage toward the workpiece so that the carriage can come to rest in any one of several second or operative positions. In certain machine tools, the apparatus comprises a cam which is driven by a variable-speed motor and can transmit motion to the carriage through the medium of a suitable follower system. It is further known to control the motor by a programming unit which can also initiate the actuation of a braking device to thus insure that the carriage will dwell in a selected second position while the respective tool removes material from the workpiece.
Since the carriage is movable forwardly exclusively by means of a cam, the machine tool must be equipped with reliable means for automatically arresting the carriage in a selected second position. The provision of such arresting means is of particular importance when the workpiece which rotates with the work spindle is to be treated by one or more tools which are mounted on the reciprocable carriage as well by one or more tools which are mounted on one or more cross slides. Stoppage of the carriage in an accurately determined position is essential when the workpiece is to be treated, in rapid sequence, first by a tool on a cross slide, thereupon by one or more tools on the carriage (e.g., on the aforementioned indexible turret which is mounted on the carriage), and thereupon again by one or more tools on a cross slide. For example, the control system of the machine must prevent a collision between the carriage and a cross slide when, following a treatment by one or more tools on the carriage, the workpiece is to be treated by a tool on the cross slide which must move relative to the carriage in order that the tool can form a cavity or recess. The situation is analogous during a change in setup, i.e., when the turret and the cross slide or cross slides are to be equipped with different tools and the persons in charge must select the extent and sequence of movements of carriage and cross slide(s). It is important in such machine tools to insure that the carriage will not overshoot its intended position, even if it is initially moved at a very high speed, as well as that a selected tool can remain in selected position without any drifting for extended periods of time. Moreover, it is desirable to insure that the carriage can leave its first or starting position without jerking or abrupt acceleration because such movement can take place while a tool on the turret is in material-removing contact with the workpiece in the revolving work spindle.
In accordance with a presently known proposal, the cam which moves the carriage forwardly is driven by a variable-speed D.C. motor which transmits motion to the cam through the medium of a two-stage gear transmission including one or more clutches and a braking device. Such machine tools further comprise a reversible multi-phase braking motor whereby the cam can receive motion from the braking motor alone, from the D.C. motor alone, or from both motors. In the latter instance, the two motors drive the cam by way of a planetary transmission. The means for selecting the operation of the drive (i.e., whether the cam is to be rotated by the one, the other or both motors) in dependency on the desired speed of forward movement of the carriage comprises a suitable programming drum with adjustable output elements in the form of angularly displaceable lobes or the like. Reference may be had to German Offenlegungsschrift No. 1,777,070.
The just described electromechanical drive for the cam exhibits a number of serious drawbacks. Thus, the overall cost of the drive (with two motors, one or more clutches, a braking device and a complex transmission) is very high. Furthermore, the control equipment which is needed to regulate the operation of such drive is so complex and bulky that it cannot be installed in or on the frame of the machine tool, i.e., it must be installed in a separate cabinet. The control equipment includes a host of relays, switches and other electronic and electrical components.